Profumo, 48, made a personal statement to the House of Commons on 22 March in which he admitted being misleading about his relationship with 21-year-old call girl Christine Keeler.

To my very deep regret I have to admit that this was not true, and that I misled you and my colleagues and the House

He had made the statement in response to allegations from fellow MPs that he was involved with Miss Keeler, who has also had relations with an attache at the Russian embassy.

It was also alleged that Profumo, who has been War Secretary since 1960, had assisted in the disappearance of Miss Keeler, who had not appeared at the Central Criminal Court where she was due to give evidence in the case against a West Indian accused of possessing a firearm.

She was later discovered in Spain.

In his letter to the Prime Minister, Profumo, said: "You will recollect that on 22 March, following certain allegations made in Parliament, I made a personal statement.

"At that time the rumour had charged me with assisting in the disappearance of a witness and with being involved in some possible breach of security.

"So serious were these charges that I allowed myself to think that my personal association with that witness, which had also been the subject of rumour, was by comparison of minor importance only.

"In my statement I said there had been no impropriety in this association.

"To my very deep regret I have to admit that this was not true, and that I misled you and my colleagues and the House."

The prime minister told Profumo that he had no option but to accept his resignation and said: "This is a great tragedy for you, you family and your friends.

Profumo also sent a personal letter to the chairman of the Conservative party in his constituency of Stratford-upon-Avon where there will now be a by-election.

Parliamentary Under-Secretary of State and Financial Secretary at the War Office, Mr James Ramsden, will take temporary charge of the department.

He entered Parliament in 1940 at the age of 25, representing Kettering.

At the time he was the youngest member of the Commons.

Before being appointed at the War Office he held many positions including Parliamentary Secretary to the Ministry of Civil Aviation, Parliamentary Under-Secretary of State for the Colonies and Minister of State at the Foreign Office.

Following his resignation he devoted himself to philanthropy and in 1975 was named Commander of the British Empire for his charitable work.